Not Applicable
Not Applicable
The field of the invention is electro-optical radiation sources and detectors, and more particularly, to an apparatus that simultaneously functions as an electro-optical radiation source and an electro-optical radiation detector.
Non-dispersive Infrared (NDIR) techniques utilizing the characteristic absorption bands of gases in the infrared have long been considered as one of the best methods for composite gas measurement. These techniques take advantage of the fact that various gases exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum. The term xe2x80x9cnon-dispersivexe2x80x9d refers to the type of apparatus incorporating this particular measurement technique, typically including a narrow band pass interference filter (as opposed to a xe2x80x9cdispersivexe2x80x9d element, such as a prism or a diffraction grating) to isolate and pass radiation in a particular wavelength band from a spectrally broad band infrared source. The gas concentration is discerned from the detected intensity modulation of source radiation that is passed by the filter coincident in wavelength with a strong absorption band of the gas to be measured.
A prior art NDIR gas analyzer typically includes a discrete infrared source with a motor-driven mechanical chopper to modulate the source so that synchronous detection may be used to discriminate spurious infrared radiation from surroundings; a pump to push gas through a sample chamber; a narrow band-pass interference filter; a sensitive infrared detector, and infrared optics/windows to focus the infrared energy from the source onto the detector. Although the NDIR gas measurement technique is recognized as one of the most effective methodologies for composite gas measurement available, it has not enjoyed wide application because of its complexity and high cost of implementation.
Infrared absorption instruments traditionally contain a source of infrared radiation, a means of spectral selection for the gas under study, an absorption cell with associated gas sample handling and/or conditioning, any necessary optics, a sensitive infrared detector, and associated signal processing electronics. A typical source of infrared radiation includes an incandescent filament or a thin film conductor. The emissions spectrum of the infrared source may be tailored via surface texturing techniques, as are described in U.S. Pat. No. 5,838,016. The invention simplifies and reduces the cost of an infrared instrument by integrating the function of the infrared source and infrared detector into a single self-supporting thin-film bolometer element. This element is packaged with inexpensive molded plastic optics and a conventional spectral filter to make a transistor-size xe2x80x9csensor engine.xe2x80x9d Combined with a simple reflector plate to define the gas sampling region, this sensor engine provides a complete gas sensor instrument which is extremely inexpensive and which will approach the sensitivity of conventional infrared absorption instruments.
The present invention is an apparatus for detecting a gas having a distinct infrared radiation absorption characteristics. The apparatus includes a spectral source/bolometer for conducting an electrical current and for producing an infrared radiation. The source/bolometer is disposed along an axis and has a temperature and a characteristic resistance; the characteristic resistance is a predetermined function of the temperature. The apparatus further includes a concentrating reflector for directing the infrared radiation along the axis, first through a spectral filter and then through the gas. The apparatus also includes a return reflector disposed along the axis beyond the spectral filter and the gas, such that at least a portion of the infrared radiation passing through the filter and the gas is reflected back through the gas and the filter to the source/bolometer. The apparatus further includes a driver/detector for driving a current through the source/bolometer, for determining the characteristic resistance, and for detecting the gas from a variation of the characteristic resistance.
In one embodiment, the source/bolometer includes a thin-film conductor.
In another embodiment, the source/bolometer includes a filament conductor.
In another embodiment, the source/bolometer includes surface texturing so as to tailor a spectral characteristic of the infrared radiation.
In a further embodiment, the concentrating reflector is disposed about the axis so as to form a first aperture along the axis and a second aperture along the axis, the source/bolometer is disposed at the first aperture and the spectral filter is disposed at the second aperture.
In another embodiment, the concentrating reflector forms a compound parabolic concentrator.
In another embodiment, the return reflector defines a gas sampling region.
In another embodiment, the return reflector includes a flat reflective surface disposed substantially perpendicular to the axis.
In another embodiment, the return reflector includes a contoured reflective surface disposed substantially about the axis.
In one embodiment, the contoured reflective surface includes a parabolic surface.
In another embodiment, the spectral filter substantially passes infrared radiation within a first passband and substantially blocks infrared radiation outside of the first passband.
In a further embodiment, the spectral filter includes a micromesh reflective filter.
In another embodiment, the micromesh reflective filter is fabricated using micro-electro-mechanical systems technology.
In yet another embodiment, the driver/detector includes a Wheatstone bridge circuit having a first resistor pair and a second resistor pair, wherein a first resistor of the first resistor pair includes the source/bolometer.
In another embodiment, a second resistor of the first resistor pair includes a blind source/bolometer being identical to the source/bolometer and filtered at a second passband.
In another embodiment, a ratio of the first resistor pair is substantially equal to a ratio of the second resistor pair.